This study is undertaken to investigate the anatomy and topography of the atrial muscle fibers in great detail. Particular attention will be paid to the apposition of individual cells and cell bundles to study the geometry of current distribution. At least one selected bundle of atrial fibers will be cut into serial sections over a 3 mm length in an effort to estimate low resistance pathways between the constituent fibers. Another part of this project involves the cutting of serial sections through the AV, and eventually, the SA node of the rabbit rt atrium in order to study the contributions that fiber and bundle geometry may make to impulse conduction and delay. Furthermore, the proposed investigations will examine the distribution of transverse tubules and of the sarcoplasmic reticulum in atrial muscle by taking advantage of lanthanum and peroxidase as extracellular labels, and of our new SR stain for the sarcoplamic reticulum. Finally, we shall look at the general morphology of atrial fibers in localized different regions. BIBLIOGRAPHIC REFERENCES: Cardiac Muscle: An Attempt to Relate Structure to Function. Page, A.W., Anderson, E.A., Johnson, Andres Manring, and Joachim R. Sommer. J. Mol. Cell. Cardiol. 1975 (accepted for publication). The Ultrastructure of the Mammalian Cardiac Muscle Cell. Joachim R. Sommer and Robert A.Waugh. Am. J. Path. (accepted for publication, Feb. 1976).